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Functional Structure/Activity Relationships

Techno-functional properties of crude extracts from the green microalga Tetraselmis suecica Edgar Suarez Garcia, Jelle van Leeuwen, Carl Safi, Lolke Sijtsma, Lambertus A.M. van den Broek, Michel H.M. Eppink, Rene H. Wijffels, and Corjan van den Berg J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b01884 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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Journal of Agricultural and Food Chemistry

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Techno-functional properties of crude extracts from the green microalga Tetraselmis suecica

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E. Suarez Garciaa,∗, J.J.A. van Leeuwenb, C. Safib, L. Sijtsmab, L.A.M. van den Broekb, M.H.M. Eppinka, R.H. Wijffelsa,c, C. van den Berga.

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a

: Bioprocess Engineering, AlgaePARC, Wageningen University and Research, PO Box 16, 6700 AA Wageningen, The Netherlands. b : Wageningen Food & Biobased Research, Wageningen University and Research, PO Box 17, 6700 AA Wageningen, The Netherlands. c : Nord University, Faculty of Biosciences and Aquaculture, N-8049 Bodø, Norway.

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∗

Corresponding author: Tel: +31 317 48 5096. Email address: [email protected] (Edgar Suarez)

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Abstract:

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A mild fractionation process to extract functional biomolecules from green

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microalgae was implemented. The process includes bead milling, centrifugation and

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filtration with several membrane cut-offs. For each fraction, the corresponding

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composition was measured and the surface activity and gelation behavior were

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determined. A maximum protein yield of 12 % was obtained in the supernatant after

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bead milling and between 3.2-11.7 % after filtration. Compared to whey protein

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isolate, most of the algae fractions exhibited comparable or enhanced functionality.

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Surface activity for air-water and oil-water interfaces, and gelation activities were

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notably superior for the retentate fractions compared to the permeates. It is proposed

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that such functionality in the retentates is due to the presence of hydrophobic

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compounds and molecular complexes exhibiting a similar behavior as Pickering

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particles. We demonstrated that excellent functionality can be obtained with crude

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fractions, requiring minimum processing and thus, constituting an interesting option

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for commercial applications.

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Key words: Crude extract; bead milling; filtration; surface activity; gelation.

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1. Introduction

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Algae have been recognized as a promising renewable feedstock for the production

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of fuels and bulk chemicals, pigments and particularly food and feed ingredients1. To

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obtain such products, an intricate series of unit operations is often needed, involving

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cell disintegration, extraction, and purification. Algae biorefinery is therefore

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associated with multiple downstream processing steps that result in several highly

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pure products2. For some applications, however, product functionality must be the

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determining criterion, rather than product purity3. The functional properties of a

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certain product (e.g., foaming, emulsification and gelation) is determined by the

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presence of proteins, carbohydrates and lipids and the interactions among them3. It is

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therefore expected that complex mixtures also show certain functionality. In other

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words, impure fractions can be potentially marketed as functional ingredients.

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The functionality of algae proteins has been investigated in several publications.

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Excellent gelling properties were observed for proteins extracted from the

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cyanobacteria Arthrospira platensis4. A soluble protein isolate (ASPI) from the

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microalga Tetraselmis sp. was prepared by Schwenzfeier et al.,5. The ASPI, containing

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64 % proteins and 24 % carbohydrates, showed complete solubility at a pH above 5.5,

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the formation of stable emulsions at pH 5-76 and superior foam stability at pH 5-7,

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compared to whey and egg protein isolates7. The authors argued that the presence of

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charged sugars contributed to the foaming and emulsifying properties of the algae

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protein isolate8. Proteins from the microalga Chlorella vulgaris were extracted after a

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process of homogenization, pH shift and ultrafiltration9. Only the permeate fractions

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obtained under neutral conditions showed higher emulsifying capacity and stability

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than commercial sodium caseinate and soy protein isolate. Similarly, water soluble

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proteins from Haematococcus pluvialis were extracted using high pressure

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homogenization, centrifugation and pH shift. The resulting supernatants were rich in

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proteins (26-44 dw %), carbohydrates and lipids, and exhibited superior emulsifying

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stability and activity index in comparison to sodium caseinate. Emulsification capacity,

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however, was lower10.

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A protein isolate (70 dw % proteins) obtained from Arthrospira platentis was

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evaluated for several functional properties11. it was found that emulsification and

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foaming are highly dependent on the pH and correlate directly with protein solubility.

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Under the presence of a plasticizer, the fractions were able to form stable gels.

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Isoelectric precipitation was applied to extract proteins from bead milled

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Nannochloropsis oculata12. the extract, containing 23 % proteins and 15 % lipids (dw)

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was proposed as an interesting functional ingredient for food and feed. Extraction and

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precipitation of proteins from Nannochloropsis spp. after thermal treatment and pH

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shift was reported by Gerde et al.,13. Although the extraction conditions were harsh,

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the authors pointed out that the high degree of glycosylation of the protein extract

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could have led to unique functional properties. Waghmare et al.,14 employed a three-

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phase system to concentrate proteins from Chlorella pyrenoidosa, and obtained a

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concentrate with 78 % (dw) proteins, exhibiting excellent foaming and good oil holding

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capacities.

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The present study presents an overview of the functional activity (surface activity and gelation behaviour) of extracts obtained under mild conditions from the marine

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microalga Tetraselmis suecica. The fractionation process consists of bead milling,

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centrifugation and filtration, which are simple and standard technologies in

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downstream processing and thus with high potential to be scalable. The effect of the

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membrane cut-off on the composition and functionality of the permeates and

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retentates was also investigated. The main objective of this research was to

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demonstrate a simple fractionation strategy to recover algae fractions and to show

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that crude extracts display comparable or superior functionality than commercial

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protein isolates.

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2. Material and methods

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2.1.

Alga cultivation and harvesting

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Tetraselmis suecica (UTEX LB2286, University of Texas Culture Collection of Algae,

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USA) was cultivated and harvested as described by Postma et al.,15. In short, the

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cultures where maintained in a greenhouse (AlgaePARC, Wageningen - The

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Netherlands) at 20 oC under 0.254 vvm (5 v% CO2) sparging gas and 373 μmol m-2 s-1 of

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continuous artificial incident light. The cultures were harvested and centrifuged and

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the resulting biomass was kept at 4 oC until further use.

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2.2.

Preparation of algae fractions

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A simple fractionation process is proposed, involving the steps of bead milling,

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centrifugation and filtration (Fig. 1). After separation, every fraction (crude extract,

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solids, permeate and retentate) was collected and analysed independently.

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Bead milling and centrifugation. Disruption experiments were conducted in a horizontal 0.075 L bead mill (Dyno-Mill Research Lab, Willy A. Bachofen AF

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Maschinenfabrik, Switzerland) operated in batch recirculation mode. Algae

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suspensions containing ̴100 g L-1 biomass were prepared in phosphate saline buffer

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pH 7 (1.54 mM KH2PO4, 2.71 mM Na2HPO4.2H2O, 155.2 mM NaCl). All runs were

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conducted for 1 h, under the same conditions as presented before15, except bead size,

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which was kept constant at 0.4 mm. Bead milled suspensions were centrifuged at

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20000 x g and 20 oC for 30 min and the supernatants and pellets were stored

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separately at -20 oC for further analysis.

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Filtration. Ultrafiltration experiments were conducted on a LabscaleTM TFF system

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(Millipore, Billerica, MA) fitted with membrane cassettes with a filtration area of 50

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cm2 and cut-offs of 300 kDa or 10 kDa (Pellicon XL Ultrafiltration Ultracell) at a fixed

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average transmembrane pressure (TMP) of 2.07 bar. Microfiltration was performed by

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manually pressing feed (crude extract) through 0.45 μm dead-end cellulose filters

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(Sartorius). Permeates and retentates were stored at -20 oC until analysis.

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2.3.

Analytical methods

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Biomass characterization. Dry weight (DW), proteins, carbohydrates and starch

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analysis were conducted as described by Postma et al.,15. To summarize, dry weight

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was estimated gravimetrically, proteins and carbohydrates were measured with the

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methods of Lowry1 and Dubois17 respectively. Total lipids were measured with the

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method of Folch18 and starch with the total starch assay kit of Megazyme®. Total ash

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was determined gravimetrically by burning a known amount of freeze dried biomass in

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an oven at 575 oC and regarding the remaining material as ash.

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Mass yields. Mass yields per component (Yi) were estimated according to:

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% =

, ,

×

Eq. 1

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Where mi is the mass of component i (protein, carbohydrates, lipids, ash and

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starch). Subscripts j and b refer to each fraction evaluated (crude extract, permeate,

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filtrate, etc.) and initial biomass, respectively.

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Acrylamide native gel electrophoresis. Protein samples were prepared in Milli-Q®

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water and diluted with native buffer (Biorad) at a ratio 1:0.8 v/v. 25 μL of the resulting

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solution were loaded per lane in a 4–20% Criterion TGX gel (Biorad). NativeMark™ (Life

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Technologies) was used as marker. Electrophoresis was run at 125 V constant for 75

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min using Tris-Glycine (Biorad) as running buffer. Staining of the gels was performed

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with Bio-Safe Coomassie blue (Biorad) for 2 h. Gels were left overnight after abundant

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washing with demineralized water to further develop the bands before scanning.

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Particle size: Particle size distributions of solutions containing algae extracts (0.1%

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w/v protein basis) were determined using a Nanosizer- Zetasizer Malvern ZEN 5600SN

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(Malvern Instruments Ltd, Malvern, UK) at room temperature and pH 7.

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2.4.

Techno functional properties

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Protein solutions. Before determining techno functional properties, samples were

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lyophilised for at least 27 h using a Sublimator 2x3x3 (Zirbus Technology GmbH) and

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stored air-tight at room temperature until use. The corresponding amount of algae

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fraction was weighted and dissolved in distilled water in order to obtain a desired

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protein concentration. The resulting solution was adjusted to pH 7 with NaOH 0.1 M

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before each analysis.

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Whey protein Isolate (WPI). Whey Protein Isolate (BiPRO, Davisco Foods

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international) containing 97.6 % protein and 2 % ash (DW) and 5 % moisture content,

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was used as reference commercial standard.

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Surface activity. The ability of the fractions to influence surface tension was

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determined by recording the interfacial tension of static controlled droplets using an

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Automated Drop Tensiometer (ADT Tracker®, Teclis Scientific, France). The ADT

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measures surface tension according to the Young-Laplace theory19. Foaming and

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emulsification behaviour were derived from static drop experiments as presented

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below. All experiments were conducted at room temperature.

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Foaming. surface activity for air-water interface was studied from air-water

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droplets. Each droplet was formed with 11 μL of suspension containing 0.1 % protein

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(w/v). The droplet was kept hanging from a needle while subjected to a stream of

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saturated air flowing vertically in a 5 mL cuvette. Surface tension of the droplet was

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recorded for a period of 36 min.

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Emulsification. Studies of surface activity for oil-water interface were conducted on

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20 μL static drop of hexadecane (Anhydrous, > 99%, Sigma Aldrich) submerged in a 5

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mL 0.1 % protein solution (w/v). Surface tension was recorded for 60 min. After

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equilibrium is reached, perturbations on the droplet’s volume were enforced by adding

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1 μL solvent, 5 times in 10 s, ensuring variations in surface area lower than 10 %.

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Fourier analysis was conducted on the dilation data, resulting in the elastic modulus є

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(mN m-1).

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Gelation. Gelation tests were conducted according to Martin et al.,20 using an

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Anton Paar MCR 302 (Modular Compact Rheometer) with a heating rate of 5 oC min-1

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in the range 25 to 95 oC.

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2.5.

Statistics

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All experiments were conducted in duplicates from independent experiments.

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Statistical analysis at 95 % confidence level were conducted using R (V 3.2.2).

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Significance was evaluated applying one way ANOVA. To compare significantly

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different means, a t-test or a Tukey’s Honest Significant Tests (HSD) was applied.

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3. Results and Discussion

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3.1.

Fractionation process.

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The complete mass balance and corresponding compositions and mass yields per

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component, according to the fractionation process depicted in Fig. 1., are presented in

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Table 1. and Fig. 2A. The carbohydrates content in the initial biomass (41 % dw) was

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significantly higher compared to the values reported by Schwenzfeier et al.,5 for

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Tetraselmis sp (24 % dw) while the protein content was similar (~ 37 % dw). The high

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content of carbohydrates can be due to the accumulation of starch granules and

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seasonal variation as noted by Michels et al.,21 for cultures of Tetraselmis suecica

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maintained in green houses.

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During bead milling, the shear caused by bead-bead collisions leads to the release

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of intracellular components. Proteins are released quickly, reaching a maximum

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concentration at short milling times. Carbohydrates, on the contrary, display a gradual

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increasing trend as noted in our previous study15. After bead milling, over 30 % of the

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total sugars and only 12 % of the proteins were found in the soluble phase or “Crude

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Extract (CE)” (Fig. 2B). This indicates than only a small fraction of the total proteins in

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T. suecica is soluble and can be extracted in the aqueous phase after complete

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mechanical disintegration. Postma et al.,15 and Schwenzfeier et al.,5 reported yields of

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soluble proteins of approximately 20 % for Tetraselmis species after bead milling. This

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higher yield can be due to differences in biomass composition and in the calculation

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method, as we are reporting mass yields according to Eq. 1. The insoluble phase

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(solids) contains almost equal proportions of proteins, carbohydrates and lipids, in

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addition to 5.3 ± 0.9 % (dw) ash (Fig. 2A). It constitutes an interesting material for the

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preparation of feed formulations for livestock, poultry and aquaculture22.

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The effect of the membrane cut-off on the fractionation yields and functionality

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was further investigated. Sequential filtration has been applied for algae

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biorefinery23,24, but the study of the functionality of the resulting fractions remains

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elusive. For each membrane cut-off, the content of proteins, carbohydrates, lipids and

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ash were quantified and the results are presented in Table 1. The corresponding mass

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yields are also given. The permeate fraction after microfiltration (0.45 μm membrane)

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resulted in the highest yields for all components. This is anticipated as this membrane

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removes only large particles, yielding a permeate containing nearly 97 % of the total

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feed. Unexpectedly, the protein yield in the permeate of the 10 kDa membrane

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doubles that of the 300 kDa. This can be due to fouling for the 300 kDa membrane,

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preventing a significant fraction of proteins to migrate to the permeate. The same

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phenomena was observed by Safi et al.,24 who noted a higher degree of membrane

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fouling for higher cut-offs and attributed this to the formation of polarization layers

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and to the adsorptive fouling during the filtration of algae suspensions. Such fouling

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can also explain why the retentate of the 300 kDa membrane shows a higher amount

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of total lipids compared with the 10 kDa.

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The permeates of the 300 and 10 kDa appeared clear, indicating complete

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removal of pigments. This was also observed in other study24 for extracts from

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Nannochloropsis gaditana using polyethersulfone membranes of 1000, 500 and 300

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kDa. Pigments are recovered in the retentate phase due mainly to the hydrophilic

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nature of the membrane materials used in both studies. Regarding starch, we

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measured concentrations in the range 1.1-1.9 g kg-1 in both the permeate and

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retentate fractions (Table 1). This contradicts the results of Safi et al.,23,24 who reported

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complete retention of starch for membranes ranging from 1000 to 10 kDa. This

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difference can be due to a lesser extent of fouling for the cellulose-based membranes

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used in our research compared to polyethersulfone-based membranes used by Safi

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and co-workers, allowing starch fragments to also migrate to the permeate phase.

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Besides a strong green colour, the retentate fractions showed a significantly

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higher content of proteins, carbohydrates and lipids (Table 1). This suggests that

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proteins are associated with pigments and polysaccharides. In fact, it has been

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reported that proteins in green microalgae are often covalently bound to lipids25,

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polysaccharides and sugars5,8,26, and pigments27, forming molecular complexes that

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can easily be retained by membranes during ultrafiltration.

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The fractionation process presented in this investigation was conducted under mild conditions (room temperature and native pH of 5.7 ± 0.2) and without the

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addition of chemicals. Native gel electrophoresis (Fig. 3) shows the expected bands for

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T. suecica15 and demonstrates that after bead milling and filtration, the main protein

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bands are maintained. In addition, it is confirmed that the permeate of the 10 kDa only

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contains low molecular weight proteins. A maximum overall total protein yield of 6.1 %

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was observed after filtration (Table 1). This is comparable with a 7 % yield reported for

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Tetraselmis sp under a process involving bead milling, dialysis, chromatography and

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precipitation5. On the contrary, Ursu et al.,9 found a yield of 87 % for Chlorella vulgaris

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after filtration over a 300 kDa membrane. The authors attributed this to the fact that

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proteins from the algae extracts exists as large macromolecular aggregates with

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molecular weights above 670 kDa, thus the majority of the proteins are retained. The

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corresponding protein yields for the filtration step in the present investigation are 40.1

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% and 26.9 % for the retentates of the 300 and 10 kDa membranes respectively. The

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lower yields are an indication of a more diverse range of proteins and macromolecular

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complexes in the extracts from T. suecica. Such diversity may lead to a richer technical

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functionality.

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3.2.

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3.2.1.

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Techno-functional properties Surface activity: foaming and emulsification

Surface activity - foaming and emulsification - refers to the ability of certain

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compounds to form and stabilize air-water (awi) or oil-water (owi) interfaces. Such

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stabilization takes place due to the formation of network-like structures around a clean

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surface, which effectively lowers its surface tension. This requires surface active

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molecules to be soluble, to diffuse to, and to adsorb on an interface28. Furthermore,

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molecular rearrangements and interactions among molecules adsorbed on the surface

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also lead to variations of the surface tension29. In this regard, functionality is not

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limited to proteins but can be enhanced by the presence and chemical nature of other

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biomolecules, and their ability to interact3.

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The dynamic surface tension (γ [mN m-1]) of samples containing extracts from T.

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suecica and whey protein isolate (WPI) as reference protein is presented in Fig. 3A for

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awi and Fig. 3B for owi. For both cases, the surface tension decreases sharply and

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reaches slowly an equilibrium level. This behaviour is typical for surface active

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molecules and reflects the basic mechanisms mentioned before. For a theoretical

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treatment of the experimental data presented in Fig 3, the reader is referred to the

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supplementary information.

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The surface activity of alga extracts in awi showed a comparable or superior

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performance than samples prepared with WPI. This can be seen in Fig. 3A by

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comparing the slopes (reflecting the rates of diffusion, adsorption and stabilization)

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and the surface tension at equilibrium conditions, which reflects the extent of surface

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activity. The retentate fractions from the 10 and 300 kDa membranes resulted in the

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strongest activities. On the contrary, the permeate from the 10 kDa membrane

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showed the poorest performance. This clearly demonstrates the effect of the

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fractionation strategy. The retentate fractions are rich in pigments and lipids, contain

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larger particles and a wider range of proteins (Fig. 4). On the contrary, the permeate

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fractions are depleted of pigments, contain only small particles and, for the case of the

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10 kDa membrane, lack of large molecular weight proteins (Fig. 4A). The presence of

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larger macromolecular complexes appear to favour the stabilization process. It is

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therefore not surprising that the alga extracts, containing a mixture of compounds,

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presented a higher activity than a pure protein isolate. This has been attributed not

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only to proteins and glycoproteins8,13,14 but to charged sugars6 and pigments4.

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The surface activity in owi presented a similar behaviour: a sharp decline of γ

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followed by a slow decrease to reach a plateau phase (Fig. 3B). All samples containing

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alga extracts showed a comparable performance as WPI. Other studies have also found

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a similar or superior emulsification activity of algae proteins in comparison with

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commercial protein isolates. The superior performance has been attributed to the

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activity of the proteins present in the extracts10, but also to the interactions and

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favourable effect of other biomolecules present in the extract such as sugars6,

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chlorophyll and lipids9. Emulsion stabilization takes place because of the development

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of steric forces around the surfaces which limit the extent of coalescence and emulsion

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degradation10.

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For the retentate fractions (10 and 300 kDa) measurements of γ could not

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continue beyond 1500 s (data not shown) due to the sudden detachment of the

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hexadecane drop from the measurement device. This suggests a remarkable surface

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stabilization, probably brought about by the presence of pigments and higher lipid

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content (Table 1) which allows a stronger interaction with the hexadecane. For the

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remaining fractions, surface activity was further studied by imposing periodic

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expansions and compressions on the droplet’s surface area (Fig. 3B). As can be seen,

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all samples recovered their original surface tension without appreciable deformation,

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indicating a high degree of stability. A measure of such stability during perturbation is

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obtained with the elastic modulus є. The values of є for samples containing alga

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extracts varied between 35-41 mNm-1 while that for WPI was nearly 40 mNm-1. This

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elastic behaviour is typical of molecules which can store energy, such as proteins30.

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Contrary to our findings, Ursu et al.,9 observed better emulsification activity for

321

the permeates of a 300 kDa (polyethersulfone) filtration process. The authors argued

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that after the extraction process proteins were denatured and formed aggregates,

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which were later recovered in the retentate fractions. Such aggregates therefore

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displayed inferior functionality. As indicated by native gel analysis (Fig. 4A), the

325

fractionation process employed in the present research did not lead to appreciable

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protein denaturation nor aggregation and thus, both fractions are enriched with

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functional molecules.

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3.2.2.

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The textural attributes of several food products result from the development of

330

stable gels. In general terms, gels are formed after a two steps process. In the first step

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the functional groups of the active molecules are exposed due to thermal or chemical

332

denaturation. Later, the exposed functional groups interact with specific regions of

333

neighbouring molecules, creating a network like structure. Furthermore, cross links are

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developed, leading to a three-dimensional assembly with specific viscoelastic

335

properties31. Only few studies have addressed the gelation properties of algae

336

proteins. In all cases, proteins from Spirulina platensis have been investigated4,11,32.

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Gelation

To study the gelation activity of alga extracts, the storage modulus G' [Pa] was measured during a defined heating – cooling profile. The storage modulus indicates

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339

the force required to deform certain material and thus, it serves as a quantitative

340

measure of the strength of the formed gels. The results are presented in Fig. 5A for

341

alga extracts prepared in this study, and in Fig. 5B comparing with data published for

342

Rubisco from spinach and two commercial protein isolates (WPI and Egg White Protein

343

EWP)20. At first, all fractions were prepared at 5 % protein content. However, due to

344

solubility constrains, only two fractions could be prepared at 10 % protein content: CE

345

and 0.45 μm P. WPI did not show any gel-like behaviour at 5 nor at 10 % protein

346

content, which was also observed by Martin et al.,20.

347

During the heating phase (25 to 95 oC) the onset of gelation (tg) marks the time at

348

which G' starts increasing, and reflects the thermal stability of the molecules in the

349

sample. Rubisco, WPI and EWP are stable at temperatures below 65 oC33, 77 oC and 84

350

o 34

C respectively and therefore long tg are expected. For instance tg of 10, 12, 15 min

351

are reported for gels formed with three different isolates containing 2.5–12.5 %

352

protein (Fig. 5B)20.

353

For alga extracts containing 10 % proteins (CE and 0.45 μm P), G' raised rapidly

354

approximately 1 min after heating was initiated, which suggests low thermal stability.

355

It appears that proteins denature quickly and readily form gels. On the contrary, for

356

samples containing 5 % proteins, the onset of gelation took place at longer times (2-15

357

min). This reflects the effect of concentration on the development of stable gels. In

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fact, gels are formed only above a critical concentration specific for each protein35. For

359

example, Rubisco can form gels at concentrations as low as 0.5 %33. On the contrary,

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Proteins from Arthrospira platensis could only form gels from 1.5-2.5 % w/w4 and 12

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%32.

362

Gels formed with 10 % proteins show a strong increase in G' followed by a plateau

363

at 95 oC (G'∞). A similar trend but with a moderate slope was observed for fractions

364

containing 5 % proteins (Fig. 5A). This is possibly due to a lower availability of

365

interacting molecules at 5 %, which reduces the probabilities of forming new bonds

366

upon heating. The values of G'∞ for all samples are presented in Table 2. Interestingly,

367

gels prepared with CE (10 % protein) showed superior gel strength after the heating

368

phase compared to 2.5 % Rubisco (Fig. 5B), 10 % soy (G'∞ = 400 Pa), and 21.5 % lupine

369

(G'∞ = 340 Pa) protein isolates20. WPI (12.5 %) and EWP (10 %) form substantially

370

stronger gels, which may be due to the prevalence of non-covalent interactions which

371

render them as rigid and brittle gels36.

372

When the samples are cooled down to room temperature, a further increase in G'

373

is observed for most fractions, except for the permeates of 300 and 10 kDa (Fig. 5A).

374

Martin et al.,20 postulates that during this phase, hydrophobic interactions and

375

hydrogen bonds are primarily responsible for the development of a stronger gel

376

network. Weak or lack of hydrophobic interactions in the permeates can indeed occur

377

due to the hydrophilic nature of the membrane materials used during fractionation.

378

When the temperature is sustained at 25 oC, a new plateau is reached (Fig. 5)

379

corresponding to G'max (maximum gel strength). In Table 2 the values of G'max are

380

tabulated. Once more, gels prepared with CE or 0.45 μm at 10 % protein registered the

381

highest values, only comparable with 10 % soy (G'max=1500 Pa) and 17.5 % pea

17



382

(G'max=3100 Pa) isolates20. Gels formed with WPI and EWP greatly surpass the strength

383

of the fractions investigated in this study (Fig. 5B).

384

3.3.

Functional activity and purity

385

We have shown that all alga extracts display a similar or superior functionality

386

compared to the commercial protein isolate WPI. In addition, we observed that the

387

retentate fractions presented better functionality compared to the crude fractions or

388

permeates. This can be due to:

389

i.

390

stabilize emulsions and to form stable gels4. The strong green colour and a higher

391

amount of total lipids (Table 1) indeed confirm that virtually all pigments are recovered

392

in the retentate phase. In addition, due the hydrophilic nature of the membrane used

393

in this research, the permeate fractions are expected to be depleted of hydrophones.

394

Under this condition, hydrophobic interactions in the permeates are limited, which in

395

turn results in a poorer functional activity.

396

ii.

397

presented in Fig. 4B, the average particle size in the retentate fractions is significantly

398

higher. Such particles correspond to large molecular aggregates or fragments of cell

399

wall, membranes and other cellular structures. Tenorio et al.,38 studied the interfacial

400

properties of thylakoid membrane fragments obtained from leaves and suggested as

401

well that their functionality resembles that of Pickering stabilizers.

402

iii.

403

enriched with cell fragments originated from the cell wall. The cell wall of Tetraselmis

Pigment-protein complexes, which in algae extracts have been found to

The permeate fractions perform as soft particles or Pickering stabilizers37. As

Divalent cations. As mentioned before, the renteate fractions are likely to be

18


Page 18 of 29

Page 19 of 29


404

species have been found to contain approximately 4 % of Ca2+ (DW)39. Divalent

405

cations, like Ca2+, contribute to the development of bridges among charged sites of

406

active molecules, therefore enhancing the strength of films around surfaces and

407

networks within gels36.

408

Besides the remarkable functional activity displayed by extracts obtained from

409

green microalgae, their potential application as food ingredients is still constrained by

410

the solubility, strong green colour, risk of off-flavour and economics. We have studied

411

samples containing 0.1, 5 and 10 % proteins, which are ranges commonly found in

412

literature. However, further exploration on how solubility is affected by pH, ionic

413

strength and concentration could provide more specific information on the possible

414

applications in foods. For specific markets and products, the characteristic colour and

415

organoleptic properties of the retentate fractions or crude extract may impede their

416

applicability. In terms of protein recovery, we have observed total yields of soluble

417

proteins of about 12 % after bead milling and 3-6 % after filtration. Although the

418

published yields of water soluble proteins varies considerably (5 - 55 %) depending on

419

the algal strain and separation method40,41 , it is clear that the recovery of functional

420

proteins from the insoluble phase is still the most important challenge.

421

The concept of functionality linked with purity and native conformation needs to

422

be critically evaluated. Waghmare et al.,14 observed excellent foam properties of the

423

protein extracts obtained after a harsh process in which proteins were mostly

424

denatured. Our research showed that excellent functionally can be obtained with

425

crude samples, even after minimal separation steps (crude extract, Fig 1). This fraction

19



426

therefore represents a more interesting option in terms of processing costs. In fact,

427

techno-functional properties can be improved by exploiting other compounds present

428

in algae, without the need of numerous purification steps. The presence of side

429

products or impurities can actually enhance activity. Carbohydrates9,4, pigments and

430

lipids4,10,38, ash36,43 and starch44, have been found to improve functional properties.

431

Even whole biomass could be used as functional ingredients for some applications45,46.

432

This in turn will result in simpler and more compact downstream processing and

433

therefore, more cost competitive processes.

434

Acknowledgements

435

This project is financed by the Applied and Engineering Sciences domain (TTW, former

436

STW) of the Dutch national science Foundation (NWO) under the project

437

AlgaePro4You, nr. 12635. The authors would like to thank Rebecca Zettwoog

438

(Wageningen Food and Biobased Research) for her contribution during the preliminary

439

phase of experiments.

440

Supporting information.

441

- Theoretical treatment of dynamic surface tension data for air water and oil water

442 443

interfaces. Fit to a mixed kinetic-adsorption model. - Strain sweep analysis of gels prepared with alga fractions.

444

445

446

20


Page 20 of 29

Page 21 of 29


447

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Franco, J. M. Novel foods with microalgal ingredients - Effect of gel setting conditions on the linear viscoelasticity of Spirulina and Haematococcus gels. J. Food Eng. 2012, 110 (2), 182–189. Guil-Guerrero, J. .; R, N.-J.; Lopez MArtinez, J. .; Campra Madrid, P.; RebollosoFuentes, M.; M. Functional properties of the biomass of three microalgal species. J. Food Eng. 2004, 65, 511–517.

590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613

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614

Fig. 1. Fractionation strategy of bead milled algae suspensions.

615

Fig. 2. (A) Dry weight composition of crude extract and solid fractions after bead milling and centrifugation and (B) corresponding mass yields (Eq. 1). Error bars represent standard deviations of four independent experiments and measurements in duplicates.

616 617

620

Fig. 3. Surface tension as function of time for (A) air-water and (B) hexadecane-water interfaces; Inner graph shows dilation responses. — WPI, ···· CE, —· 0.45 μm P, —·· 300 kDa R, —·· 300 kDa P, — — 10 kDa R, — — 10 kDa P. (R=Retentate, P=Permeate).

621 622 623 624

Fig. 4. A) Native gel analysis of fractions before and after filtration (M: Marker. CE: Crude Extract. R: Retentate. P: Permeate. Mi: 0.45 μm microfiltration. F3: 300 kDa filtration. F1: 10 kDa filtration). Dotted arrow and square indicates the expected band of Rubisco (~ 540 kDa). B) Average particle size for several alga fractions after Ultrafiltration.

625 626 627 628

Fig. 5. A) Storage modulus (G’ [Pa]) as function of time and temperature (—) for algae fractions: ···· CE 10 %, —· 0.45 μm P 10 % , ···· CE 5 %, —·· 300 kDa R 5 %, —·· 300 kDa P 5 %, — — 10 kDa R 5 %, — — 10 kDa P 5 %). B) comparison of algae fractions: □□□□ CE 10 %, ○○○○ 0.45 μm P 10 %, and commercial proteins20: − · − 2.5% RuBisCO, — 10% EWP, - - - 12.5% WPI.

618 619

629 630

25



631 632 633

Page 26 of 29

-1

Table 1. Compositions [g kg ] of fractions after bead milling and filtration. The data presented is the average of duplicates and corresponding standard deviations. Values in parenthesis indicate mass yields according to Eq. 1. Low case and capital letters show significantly equal means -per compound- for permeates and retentates respectively (p < 0.05). Filtration

Bead milling Feed

0.45 μm Pellet

Supernatant

300 kDa

Permeate

Permeate

a,b

Dry mass

129.2 ± 6.6

156.5 ± 10.5

62.1 ± 7.3

52.7 ± 0.2

45.4 ± 4.4

Protein

43.4 ± 2.0

58.9 ± 9.1 (77.7)

12.1 ± 0.7 (11.9)

12.2 ± 0.7 (11.7) a

Carbohydrates

47.2 ± 4.7

50.6 ± 11.7 (61.4)

34.3 ± 1.5 (31.0)

21.4 ± 1.0 (18.9)

Lipids

28.2 ± 2.8

47.3 ± 8.2 (96.1)

2.9 ± 1.3 (4.4)

2.3 ± 0.7 (3.4)

a

a

Retentate

a

97.5 ± 9.9

3.9 ± 1.2 (3.2) a,b

(1.9)

Ash

10.3 ± 1.2

8.4 ± 1.4 (46.3)

9.8 ± 1.6 (40.4)

11.9 ± 0.9 (47.8)

6.9 ± 1.1 (23.5)

Starch

25.8 ± 1.2

8.9 ± 3.3 (19.9)

1.0 ± 0.6 (1.6)

1.5 ± 0.2 (2.5)

a

1.7 ± 0.2 (2.3)

55.0 ± 0.2

A

7.8 ± 0.5 (6.1)

A

24.6 ± 5.9 (18.2)

b

a

55.4 ± 7.8 (11.4)

26.1 ± 2.5 (18.9)

7.2 ± 3.0 (2.5)

1.1 ± 0.2 (1.3)

A

10.1 ± 1.0 (9.6)

a

Permeate

A

19.6 ± 3.2 (4.4)

a

1.5 ± 0.8

10 kDa

A

1.1 ± 0.7 (0.4)

b

a

Retentate 94.9 ± 0.6

A A

17.3 ± 1.9 (3.4) A

54.5 ± 5.2 (9.9) 3.2 ± 0.7 (1.0) A

10.2 ± 0.1 (33.8)

9.0 ± 1.8 (7.5)

a

1.9 ± 0.2 (0.6)

1.7 ± 0.1 (2.2)

A

634

635 636

Table 2. Values of tg, G’∞, G’max and corresponding standard deviations for several alga fractions. Letters show significantly different means (p < 0.05) according to t-test (samples at 10%) and Tukey’s HSD test (samples at 5%). Sample

tg [min]

G'∞ [Pa]

G'max [Pa]

CE 10%

1.2 ± 0.0

0.45 μm P 10%

1.5 ± 2.0

CE 5%

14.5 ± 0.8

300 kDa R 5%

2.0 ± 1.2a

10 kDa R 5%

8.8 ± 6.6

862.5 ± 133.6 723.5 ± 125.2 3.4 a ± 0.6 26.1 a ± 4.2 8.6 a ± 0.6

2985.0a ± 473.8 2430.0 ± 424.3 4.5 a ± 1.1 145.5 a ± 19.1 60.0 a ± 9.4

637

26


Page 27 of 29


638

Fig. 1.

639 640 641

Fig. 2.

642 A

B

643 644 645 646

Fig. 3. B

A

647 648 649

27



650

Page 28 of 29

Fig. 4. B

A

651 652 653 654

Fig. 5. B

A

655

28


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